1. Field of the Invention
The invention relates generally to mechanical drives for providing precision motion; and more specifically to bearings, couplings or other features that are stabilized magnetically, for use in such drives.
2. Related Art
Both of my earlier patent documents identified above teach use, in mechanical drives, of couplings or bearings that typically transmit linear motion along a drive direction. These couplings absorb lateral motions through rolling action of balls between coupling or bearing elements.
At least one of these elements is a magnet that retains the balls between the elements--and in some configurations helps keep the elements in line. The magnet also creates compressive constraint along the drive direction; this constraint prevents backlash.
The first of my two earlier patent documents identified above relates particularly to drives in which linear motion along the drive direction is derived from rotary motion about an axis parallel to that same drive direction. The second document relates to specific coupling configurations that typically transmit torque as well as longitudinal linear motion.
The present document is not limited either to the context of rotary drives or to the transmission of torque. For brevity and simplicity in this document some terminology is used in a manner that may be partially specialized:
The word "bearings" encompasses couplings that may sometimes operate in tension, as well as bearings per se. PA1 The word "wobble" encompasses any spurious lateral motion such as lateral vibration, lateral play, and lateral jitter, as well as motions that are generated through rotation of a related shaft or screw and therefore perhaps more classically identifiable as wobble. PA1 U.S. Pat. No. 3,720,849 Bardocz PA1 U.S. Pat. No. 5,407,519 Joffe et al. PA1 U.S. Pat. No. 5,380,095 Pryor PA1 U.S. Pat. No. 5,237,238 Berghaus PA1 U.S. Pat. No. 5,001,351 Boksem.
Accordingly, the lateral-motion-absorbing devices of my earlier patent documents as well as this one may be conveniently called "wobble-absorbing magnetic bearings", or "WAM" bearings--or simply "WAMBs". In this document, reference to such WAM bearings encompasses the varieties disclosed in those earlier patent documents as well as those disclosed here.
My earlier patent documents discuss an invention of Norris, a bearing with ferromagnetic balls that are held in place without a bearing spacer or bearing retaining-ring holder by making one of the bearing surfaces magnetic. Norris's bearing is not a wobble-absorbing drive bearing.
In addition to the art cited in, and in connection with prosecution of, my above-identified earlier patent documents, I have noted the following materials which may be of interest:
While dealing primarily with improving the positioning precision of a ball-mounted moving table through magnetic constraints, Bardocz does mention that backlash along a drive direction can be removed through magnetic constraint.
The Pryor patent may be truly termed the Pryor art, but by virtue of the earlier filing date of my '743 application the Pryor art is not prior art with respect to that part of the subject matter herein which is disclosed in my U.S. Pat. No. 5,331,861. Pryor too relates to magnetic constraint of moving tables, and analogous modules such as drawer slides, rather than drives; and as he says at the outset he is not concerned with extremely high precision.
Pryor uses individual balls that either slip in setscrew ball nests and roll on opposing surfaces, or bind in the nests and slip on the opposing surfaces, or slip both on the nests and on the opposing surfaces. None of Pryor's ball elements is fully rolling--i. e., able to roll at both sides of its interface.
Wobble in drive mechanisms can have both translational and rotational components. It is a problem in many types of drives, including the rotary-to-linear converters mentioned above, because it causes small but significant errors in the work process being performed.
FIGS. 1 and 2 illustrate representative errors in a particular type of linear drive that derives linear motion from rotary motion about an axis that is transverse, not parallel, to the drive-direction axis. Analogous errors will be found in rack-and-pinion or cable drives and virtually every other type of drive, including many that involve no rotation at all--for example magnetic, pneumatic, hydraulic and cam drives; and trigonometric and other bar linkages.
FIG. 1 shows a linear friction driver in which a servo-motor 214 mounts to a base 200 and rotates a motor shaft 211. A friction wheel 212 biases 213 a drive bar 210 against the motor shaft 211 so that operation of the motor 214 in either direction impels the drive bar 210 so as to move an attached object 218 such as a stage of a table.
The drive bar 210 is assumed to be straight, although in fact every physical object necessarily has some imperfections such as the sinuosity shown with great exaggeration in FIG. 2. The object 218 is assumed to be guided by a guideway 215.
As the guide bar 210 undergoes nominally pure longitudinal displacements .DELTA.x, as for example to positions 210' shown in the broken line, the object 218 is correspondingly displaced as for example to positions 218'. Deviations from straightness in the drive bar 210 introduce lateral displacements .DELTA.y, which are undesired as they degrade the precision of whatever process is the overall purpose of the system.
For instance suppose that the table supports a mechanical part to be machined, or an electronic chip on which multiple layers are being formed photolithographically, or a position-sensitive scientific measuring instrument. The machined surface or some chip layers or scientific measurements will be wrong by the lateral displacement .DELTA.y.
Efforts to eliminate such errors commonly take the form of (1) increasing the strength of the drive bar--which mainly has the effect of damaging the guideway 215 and increasing the overall weight, bulk and cost of the system--or (2) increasing the bias against the guideway, which mainly has the effect of damaging the guide bar 210 and aggravating the problem.
As FIG. 2 shows, a drive element in the course of its action may undergo spurious rotary motion too: in effect the drive system may be waving a drive bar 210, rotating it about a center near some controlling element e.g. 211-212. Furthermore such rotary motion is not most-typically limited to motion in a single plane (such as the plane of the drawing in FIG. 2), or particularly any plane that can be identified in advance.
The situation shown in FIG. 2 is also simplified in that like sources of error are often present in mechanisms associated with movements of the driven object 218. Therefore in particular rotary mismatches between motions of the drive bar 210 and driven object 218 may be compound--i. e., rotations about more than one center.
Analogous limitations will be found in every type of drive. As will be seen in detail later, these sometimes take the form of imperfections in a guide surface (such as 215), which can reflect back along the drive train to cause inaccuracy or damage.
All such drawbacks represent a pervasive problem in the art of mechanical drives.